yum install system-config-firewall
Loaded plugins: rhnplugin, security
This system is not registered with ULN.
ULN support will be disabled.
Setting up Install Process
No package system-config-firewall available.
Nothing to do

yum install system-config-firewall
Loaded plugins: rhnplugin, security
This system is not registered with ULN.
ULN support will be disabled.
Setting up Install Process
No package system-config-firewall available.
Nothing to do

It is not necessary to edit the file and to enable any channels to apply system updates and install additional packages. The el5_latest repository is enabled by default and that's all you need to have enabled. And eventually ol5_UEK_latest if you use the UEK 2 kernel. On March 22, 2012 Oracle included updates and errata for free.

If the OP does not want to install any system updates and remain at the distribution level (s)he needs to enable only ol5_u8_base.